At first life only existed in the ocean. And they too were only single celled creatures which didn't leave much of a fossil record. But we do have evidence that later they evolved into soft bodied multicellular creatures. This vast span of evolutionary time is known as the Cryptozoic which means "the time of hidden life". The end of the Precambrian period and the start of a proper fossil record (590 million years ago) is marked by an event called the "Cambrian Explosion".
At the beginning of the Cambrian period evolution had perfected the hard shell. These shells were either made of mineral calcite absorbed from the seawater and laid down as a living shell or made of organic compounds made into a natural plastic called chitin .
Animals before this had been hunting and eating each other. But now some animals could defend themselves. So the hunters also developed new structures and techniques to get the prey.
By the end of the Cambrian period this vast array of animals dwindled and only a few evolutionary lines continued till the present day. Another result of developing shells is that they left a better fossil record. The history from Cambrian to the Phanerozoic meaning "obvious life" ( present day) is well documented .
Among the animals that survived were worm like creatures with a nervous system running down their body as well as jointed framework . The brain was at the front protected by a box the mouth and sensory organs were also at the front. Simple animals like this evolved into the first vertebrates.
First vertebrates
Fish were the first vertebrates and they arrived during Ordovician and Silurian times(505 to 408 million years ago). The first fish were known as jawless fish. Who had a sucker instead of jaws and a fin along the underside of their tail ensured that their head stayed down . They probably ate nutritious debris from the sea floor as they swam along. They were very similar to modern lamprey.
The above pictures show modern lamprey.
Proper jaws and a better skeleton was then developed. The first skeletons were not made of bones but of cartilage. The cartilaginous fish are represented by rays and sharks today.They appeared in Silurian times.
Shown above are a shark and a ray .
Next came the evolution of bone around the cartilage framework. Bone formed the skeleton and also the armour plates for protection. Then the scales that exist today came into existence. These bony armoured fish as well as the scaly fish came into existence in the Devonian period( 408-360 years ago).By this time there were so many different fish that the Devonian period was often called the "age of the fishes".
Changes on land
In the time of the Cambrian period the atmosphere was full of gasses harmful to life. That is why Early life evolved in the sea. Gradually the by-products of the early living system mixed with the atmosphere causing a build up of oxygen making land habitable. When plants started growing on land animals too started venturing out onto land.
A particular type of fish evolved muscular fins that enabled them to crawl on land as well as swim. Animal life was not permanent on land until plants took a strong foothold. Till then it seems as if they only went on explorations.
Life on land
Though there is evidence of tentative explorations in Ordovician and Cambrian times , it seems animals did not start living on land until plants had a strong foothold. In the Silurian times insects were the first to start inhabiting the plants that started growing on the banks of streams. The first fish ventured into land during Devonian times.
Why did the arthropods decide to come on land?
Theories are,
- Fauna that had established itself among the plants was too tempting of a food source to ignore.
- As an emergency measure(incase it got trapped in a drying puddle of water, etc.)
- To escape the danger of predators( there were clawed arthropods as big as alligators during that time)
Eusthenopteron was one of the first arthropods that arrived on land. They had developed lungs that could directly take oxygen unlike gills and fins with muscular lobes, supported by a network of bones, with fin material forming fringes on the edges. Two pairs were on the underside of the body allowing them to locomote in both water and land.
Eusthenopteron
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The first amphibians
By the end of the Devonian period amphibian like creatures started forming Ich
thyostega was among the first. Unlike the lobe finned fish they were clearly jointed with leg and toe bones. It seems thy had evolved foe wading through weeds in shallow water but they are effective for clambering on land as well.
They looked quite odd by modern standards as they had eight toes and still had a fish head and tail. They had to return to water to breed bit this tie to the sea was severed by the amniotes name after the amnion( membrane that contained the developing young within the egg.) and hard shelled eggs evolved. early examples include Westlothiana from Scotland and Hylonomus from Nova Scotia both dating from the Carboniferous period.
Skeliton of Ichthyostega
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The first reptiles
The simplest way of classifying the evolving of reptiles are the number of holes in their skulls behind the eye sockets.
- Anapsids-0
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- solid roof of bone behind the eye sockets.
- Prominent in Permian times as a group called pareiasaurs.
Modern relatives of pareiasaurs-
Turtle
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Tortoise
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2. Synapsids- a single hole on each side of the skull
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- Mammal-like reptiles, the major group of the Permian period.
- Died away during the Triassic and did not come back into prominence until the Teritiary period(65mya-present)
3. Diapsids- 2 holes behind the eye sockets.
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- Modern diapsids- snakes, crocodiles, lizards
Lizard-Discovery
Crocodile-wikipedia
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nake-Wildlife SOS- Mesozoic Diapsids-Dinosaurs
Encyclopedia Britannica
Turtle
Tortoise
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